CARS signals: phase matching, transverse modes, and optical damage effects

The transverse modes of the lasers used in coherent anti-Stokes Raman spectroscopy (CARS) affect the signal strength, spatial resolution, and noise. A general formulation for CARS signal calculations with arbitrary TEM modes and 3-D phase matching is presented. Numerical calculations for useful beam geometries are reported and compared with calculations based on simple models. The variation of intensity and phase across a laser beam is shown to have significant effect on the CARS signal strength. The effects of the transverse modes on window damage are discussed. Estimates of CARS signal strengths are presented for nitrogen thermometry near windows.